The invention relates to antennas and, more particularly, to direction finding or radiolocation antennas and methods of radio direction finding as well as to radio tracking and radiolocation systems in general and to scanning for target signals within a band of frequencies.
Radio tracking, direction finding or radiolocation apparatus and technology are used for military and civilian uses, including for military and civilian rescue missions, military and security control and tracking and radiolocation missions, military and civilian police operations and for various other purposes where one desires to both quickly and precisely respond to targets or other sources of radio frequency (RF) and to pinpoint their location or their precise bearing relative to RF receiving apparatus.
Heretofore, there have been many antenna configurations and radio direction finding and radiolocation techniques and systems using specialized antennas. Chief among the requirements in the design of such devices and systems is that of accuracy in direction finding. In azimuthal direction finding, it is desired to identify with precision the azimuthal direction to a source of RF energy. When triangulation of RF sources is used for radiolocation of such sources, accuracy can be increased substantially. As used for emergency location, as in the radiolocation of downed aircraft, obtaining azimuth bearings to an RF source from different locations enhances the likelihood of successful location. But in combat situations, where a downed airman may be unable to transmit for long periods, whether by reason of avoiding detection by hostile forces or because of limited transmitting power reserve, or in high security operations including clandestine tracking missions where decreasing length of transmitting time reduces risk of detection by those engaged in possible criminal activities, so that the time of transmission necessarily must be limited and speed of responding by tracking, direction finding or radiolocation system must be rapid.
It will thus be evident that increasing the speed of detection of a tracking, direction finding or radiolocation finding antenna system allows marked reduction in the time of transmission required for transmitting.
In a context in which there must be tracking, direction finding or radiolocation of multiple targets, increasing the speed of detection of a direction finding antenna system opens the possibility to determining the direction and location of greater numbers of multiple targets.
In the United States, regulatory requirement for cellular telephone system user location for emergency purposes has been established. Regulations have been promulgated by licensing authority functions of government which, in effect, impose on operators of cellular telephone systems a requirement that cellular systems provide capability for location of cellular users making use of such cellular systems.
In such situations, it is desired to carry out radiolocation with extreme speed, yet without diminution in accuracy.
Heretofore, radio direction finding as been carried out by a multitude of techniques, including amplitude nulling, signal comparison, and by triangulation methods as well as by IFF (from the term, Identify Friend-Or-Foe) protocols, as have been employed for many decades, as secondary radar co-aligned with primary radar in air commerce and as also extensively long used in military applications, coupled with the use of transponders which return encoded and/or encrypted signals, and which may employ digital protocols, in response to transmitted interrogation signals.
Many antenna arrays and configurations such as Yagi-Uda arrays and log-periodic or quasi-log-periodic arrays are known which have useful forward gain and directivity can be employed for receiving or transmitting signals relative to a known direction but their use heretofore has typically involved the use of arrangements for rotating such arrays, whether used alone, or in stacked configuration, or in phased relation, so as to selectively physically orient the array or arrays as by beam or mast rotation. Such physical orientation of arrays is not conducive to high-speed radiolocation, radio direction finding or scanning.
It has also been known to use so-called curtain antennas, which may include phased elements with selectively switchable reflectors to allow beam alignment, but the physical limitations and lack of mobility of such curtain antennas conducive to economical, highly mobile, field-usable, broadband high-speed radiolocation, radio direction finding or scanning.
Phased arrays have also been used extensively for radar, being particularly useful for target acquisition and tracking in military and aircraft uses, particularly at gigahertz frequencies, but the complication, cost and control complexity of such arrangements are typically not well-suited for cost-effective, readily portable or mobile, field-usable, broadband high-speed radiolocation, radio direction finding and scanning and particularly in VHF bands and low UHF bands.
Log-periodic antenna configurations and theory are of particular interest for adaptation to the presently inventive array, as log-periodic (LP) antenna structures offer the promise of wide frequency range if not frequency-independent (FI) consideration over a wide range of frequencies. Log-periodic antenna (LPA) theory and design is well-treated in the literature, and the characteristics of LPA constructions and quasi-log-periodic (QLP)variations thereof are succinctly and elegantly treated in P. E. Mayes, Proc. IEEE, vol. 80, no. 1, January 1992 (xe2x80x9cMayesxe2x80x9d herein). In this important paper, portions of which originally appeared in Y. T. Lo and S. W. Lee, Antenna Handbook, Theory, Applications, and Design. New York: Van Nostrand Reinhold, 1988, cited infra. Prof. Mayes cautions that the term frequency-independent (FI) is reserved for antennas that have no theoretical limitation on the bandwidth of operation. He observes that, practically, for such antennas performance cannot be even approximately constant for all frequencies for, as he points, out, there are physical bounds that limit the band over which the performance can be held almost constant. He observes that between the band limits the performance varies in a manner periodic with the logarithm of the frequency, so that these antennas are often called logarithmically periodic or log-periodic (LP) antennas. To be considered FI, Dr. Mayes instructs that for such antenna structures variation with frequency of all pertinent measures of electrical performance must be negligible between band limits that can be very widely spaced, even 100:1 or more. However, he points out that useful performance over a narrower, although very broad, band may be achieved after relaxing some structural requirements related to true frequency independence, adding that in some cases the resulting antennas perform the same as true FI antennas, but only over a limited bandwidth that cannot be easily extended. In other cases, he observes that performance may be noticeably affected by changing frequency over the operating band. These changes may not be deleterious, he allows, noting that they may even be advantageous in certain applications. Antennas with minor departures from the geometric requirements for FI performance are denoted by Mayes as being sometimes called quasi-log-periodic (QLP).
In the presently disclosed system, novel log-periodic antenna printed circuit structures are employed for forming an active high density multi-element antenna system of the invention, wherein elements formed in the array may be considered to be in sets of elements within the array, and where the elements specifically in dipolar form oriented for vertical polarization, but insofar as the number of elements, geometry, spacing, and relation to other elements within the array as well as in relation to electrical isolation elements within the array, constitute deliberate departures from true FI configuration, being suited for operation over a wide band of frequencies, elements of arrays of the invention may be more strictly regarded as QLP in design. For the purposes of this application, the term log-periodic as applied to the present invention is deemed also to include the term quasi-log-periodic, but a further distinction herein is not believed necessary.
In general terms the geometry of an FI antenna is viewed by Mayes as consisting of multiple adjoining cells, each being scaled in dimensions relative to the adjacent cell by a factor that remains fixed throughout the structure. According to the theory elucidated by Mayes, the cells may be two- or three-dimensional. If Dn represents some dimension of the nth cell, and Dn+1 is the corresponding dimension of the (n+1)th cell, then the relation             D              n        +        1                    D      n        =  r
holds according to Mayes for all applicable values of the integer n. The constant r is called by Mayes the scale factor, and Mayes notes that lower values of the index n usually refer to the larger cells, so that r less than 1.
Mayes recognizes dipole log-periodic antennas as a subset of log-periodic antennas. The theory of log-periodic dipole arrays may be further understood not only from Mayes but also from the following literature preceding Mayes and thus of not greater temporal significance or relation to the present disclosure:
[1] E. C. Jordan, G. A. Deschamps, J. D. Dyson, and P. E. Mayes, xe2x80x9cDevelopments in broadband antennas,xe2x80x9d IEEE Spectrum, vol. 1, pp. 58-71, April 1964.
[2] D. E. Isbell, xe2x80x9cNon-periodic dipole arrays,xe2x80x9d IRE Trans. Antennas Propagat., vol. AP-8, pp. 260-267, May 1960.
[3] R. L. Carrel, xe2x80x9cAnalysis and design of the log-periodic dipole array,xe2x80x9d Tech. Rep. 52, Univ. Illinois Antenna Lab., Contract AF33(616)-60719, October 1961.
[4] Y. T. Lo and S. W. Lee, Antenna Handbook, Theory, Applications, and Design. New York: Van Nostrand Reinhold, 1988.
[5] W. M. Cheong and R. W. P. King, xe2x80x9cLog-periodic dipole antenna,xe2x80x9d Radio Sci., vol. 2, pp. 1315-1325, November 1967.
[6] J. Wolter, xe2x80x9cSolution of Maxwell""s equations for log-periodic antennas,xe2x80x9d IEEE Trans. Antennas Propagat., vol. AP-18, pp. 734-741, November 1970.
[7] G. DeVito and G. B. Stracce xe2x80x9cFurther comments on the design of log-periodic dipole antennas,xe2x80x9d IEEE Trans. Antennas Propagat., vol. AP-21, pp. 303-308, May 1973.
[8] G. DeVito and G. B. Stracce, xe2x80x9cFurther comments on the design of log-periodic dipole antennas,xe2x80x9d IEEE Trans. Antennas Propagat., vol. AP-22, pp. 714-718, September 1974.
[9] P. C. Butson and G. T. Thompson, xe2x80x9cA note on the calculation of the gain of log-periodic dipole antennas,xe2x80x9d IEEE Trans. Antennas Propagat., vol. AP-24, pp. 105-106, January 1976.
Mayes treats the theory of log-periodic dipole (LPD) antenna arrays. For purposes of comparative theoretical analysis, the Mayes terminology xe2x80x9cLPD arrayxe2x80x9d is herein used in the same general sense as an antenna unit, herein designated AS, of the present invention. Such a comparison is merely used for convenience to understand the electrical relationship of antenna units of the present invention. An LPD array is depicted in FIG. 10 according to the symbology and taxonomy of Mayes, wherein the element geometry can be described by the scale factor, r, and the angle, xcex1, between the centerline and the tips of the dipoles. The definition of r used by Carrel [reference 3, supra] is, as characterized by Mayes, the ratio of the lengths of two adjacent dipoles; but the feeder between adjacent dipoles of an LPD is transposed. Hence, each cell of such a Mayes LPD may be considered to contain a pair of dipoles. Phasing associated with transposed feedline conductors is termed necessary by Mayes to produce FI performance.
Such LPD arrays can be analyzed, according to theory of Mayes, by separating the dipoles and transmission line, i.e., feedline, from which signals are either received from or transmitted to the LPD array. The terminal properties of an N-element network of dipoles can be represented by an Nxc3x97N impedance matrix. Carrel [reference 3, supra] used the formulas of the induced EMF method to calculate the impedance matrix; while according to Mayes, later work [references 5 through 8, supra] used moment methods, Mayes takes the view that results of the several techniques are not appreciably different [reference 9, supra]. Computed amplitude and phase of the terminal currents in the several dipoles of an LPD array are shown by Mayes in FIG. 11. The input currents of a few dipoles near the half-wave resonant length are significantly larger than those of any of the others. These dipoles with highest currents are collectively called by Mayes the xe2x80x9cactive regionxe2x80x9d and produce most of the radiated field; or in the present context, in which duality of receiving and transmitting and be said to exist for purposes of analysis, may be considered those elements which most greatly contribute received RF.
When frequency is changed, the active region will move along the axis of the LPD. The dimensions in wavelengths of the active region remain almost constant. Hence, within the operating band the radiation pattern is insensitive to the frequency changes. The pattern will begin to change when the active region encounters either of the two truncation points. The band of F1 patterns is, therefore, somewhat less than the ratio of the longest-to-shortest dipole lengths.
The present invention is concerned with precision in radiolocation, scanning and direction finding by the use of a receiving antenna system, and may be used for radiolocation of a wide variety of transmitting signal sources, whether modulated or not, and whether encoded or encrypted or not.
Among the several objects, features and advantages of the invention may be noted the provision of an advantageous system for scanning, radiolocation, and direction finding relative to single or multiple target transmitters, i.e., sources of RF emission, whether identified or unidentified; which system is which cost-effective, readily portable or mobile, field-usable, and broadband in operation; which operations with high speed scanning techniques to provide precise radiolocation and/or radio direction finding and scanning and tracking; and which is particularly well-suited for economical and effective use at VHF and UHF frequencies; which can be used with different types of host systems or may provide radiolocation data, as well as other information, including demodulated data from target sources, for various purposes.
The presently proposed system relates to an controlled active high density multi- element directional array for providing scanning, radio tracking, direction finding and/or radiolocation. Apparatus and methodology of the invention is useful for military and civilian uses, including rescue missions, military and security control and tracking and radiolocation missions, and for other military and civilian police operations.
The new system can be used for various other purposes where it is desired to both quickly and precisely respond to targets or other sources of RF and to pinpoint their location or their precise bearing relative to RF receiving apparatus.
Among the important attributes of the new system is its accuracy in direction finding and radiolocation in providing capability to identify with high degree of precision the azimuthal direction to a target source of RF energy.
When triangulation of target RF sources is desirable for radiolocation of such sources, accuracy can be increased substantially by the new system, with enhancement of the likelihood of precise location, such may enable tactical or emergency teams to move with speed and precision to the location of a target transmitter.
The new system is useful in combat situations in which a downed airman or crew may be unable to transmit for long periods, whether by reason of avoiding detection by hostile forces or because of limited transmitting power reserve.
Further the proposed system may be employed in high security operations including clandestine tracking missions where decreasing length of transmitting time reduces risk of detection by those engaged in possible criminal activities, so that the time of transmission necessarily must be limited. In such uses, the new system offers high speed of response by microprocessor implemented and/or host system initiated tracking, direction finding or radiolocation system to achieve rapid reporting of radiolocation and other data.
The new system allows marked reduction in the time of transmission required for transmitting in such situations as, for example, in tactical and rescue missions.
The system described herein facilitates tracking, direction finding or radiolocation of multiple targets, increasing the speed of detection of a direction finding antenna system; and thus the system allows for determination of direction and location of greater numbers of multiple targets.
The inventive system may be employed in meeting regulatory requirements in the USA and elsewhere for cellular telephone system user location for emergency purposes has been established. Thus, the inventive system can be used at cellular telephone sites at which antenna units of the present system are to be co-located with cellular site transmission and reception facilities to provide a high-accuracy capability of locating and tracking cellular users. Each of the several possible cellular sites (xe2x80x9ccellsxe2x80x9d) of a cellular system may be equipped with a system of the presently proposed type for allowing r, xcex8 location determination tracking of cellular users so as to provide the cellular system with the capability of identifying the location of each user, and extracting the position in the form of location data. In the foregoing notation, the terms r, xcex8 connote distance and azimuth location data of a target transmitter source relative to the controlled antenna array of the new system.
In such uses, the location data is then made available by the inventive new system to a host system such as a cellular communication system so as to be shared with multiple cell sites and reported centrally as for locating to police or emergency equipment the location of the user of cellular equipment.
In these various uses and situations, the new system carries out scanning and radiolocation of targets, and also permits tracking of targets with extreme speed, yet without diminution in accuracy, as for use by government and non-government agencies, and by military or strategic operations, detective and investigative agencies, and law enforcement, and in both covert or overt operations. In the commercial segment, the system has potential use for identifying the location of pieces of equipment, e.g., rail units, mobile units, or vehicles carrying specialized or sensitive cargo.
The present invention provides a novel low-loss, digitally-controlled system for providing a high speed omnidirectional, high accuracy radiolocation and precise radio direction scanning, finding. The system includes an active high density multi-element directional controlled antenna array of components configured as sectored sets of log-periodic antenna units under the control of an array control system which in turn may be under the control of a host system. The controlled antenna array is operated under the command of the array control system in such a way as to be able to radiolocate with a high degree of precision one or more possible target transmitters, i.e., target sources of RF by using circular scanning of extreme speed.
The present invention employs antenna elements having a log-periodic arrayed relationship and may alternatively employ antenna elements configured as according to Yagi-Uda arrays, sometimes referred to as Yagi arrays.
The sources may represent mobile or fixed units, persons, cellular telephone users, transmitters used for tracking, security or telemetry purposes, and myriad possible sources of RF energy capable of being received and used by the system and which it is desired to radiolocate of otherwise track by means of the system
Each antenna unit is capable of being switched in and out by PIN diodes under microprocessor control of array control system of the overall system, to provide direction finding, radiolocation or scanning with substantial antenna gain over a wide band of frequencies, such as from the order of 300 MHz to 3000 MHz, and specifically especially useful over the range of about 450 MHz to 1000 MHz which includes broad sectors of commercial radio frequencies as well as cellular telephone frequencies.
Briefly, the invention more formally comprises, or consists of, or consists essentially of, an active high density multi-element antenna system for radiolocation by identification of the direction of RF radiation from one or more target sources, wherein the system includes antenna elements configured in multiple rings about a central axis to define arcuate sectors about the central axis, each of the rings including a plurality of adjacent discrete antenna elements within each sector, and each of the elements in the concentric rings have mutual relation to corresponding elements in at least one other of the concentric rings, such that corresponding elements in the multiple rings define respective antenna units which are radial to the central axis. Each of the antenna units is selectively operatively enabled or selectively operatively disabled in rotational sequence about the central axis to provide signal selectivity in a narrow beam pattern or width defined by the respective antenna units and whereby the selectivity is rotatable about the central axis. Means is preferably provided for electrically isolating each of the antenna units from adjacent antenna units.
Method aspects of the invention are also disclosed, and are treated more fully after discussion of the system and its features and use.
Feedpoints for each of the antenna units for receiving received RF signals from the respective antenna units when enabled, and provide such received signals to an array control system.
The array control system is operated, as under microprocessor supervision as initiated, for example, by a host system, for causing RF scanning by controllably enabling and disabling the antenna units. The RF scanning of the antenna units is thus carried out about the central axis. It is also carried out within a least a selected one of the sectors, the array means being operable for identifying which antenna unit within the selected sector is representative of an azimuth representing the direction to the target source.
As will become apparent, both amplitude and phase comparison modes are used in the scanning and radiolocation operations provided by the new system.
From a method perspective, the invention more formally comprises, or consists of, or consists essentially of a method of scanning and radiolocation of one or more target sources of RF radiation comprising providing a high density multi-element antenna system having antenna elements configured in multiple rings about a central axis to define arcuate sectors about the central axis, each of the rings including a plurality of adjacent discrete antenna elements within each sector, and each of the elements in the concentric rings have mutual relation to corresponding elements in at least one other of the concentric rings, such that corresponding elements in the multiple rings define respective antenna units which are radial to the central axis, and an array control system therefor, and operating the control system to provide at least:
first mode scanning of the antenna units about the central axis in a received RF amplitude sensing mode within a least a selected one or more of the sectors selectively operatively enabling or selectively operatively disabling the antenna units in rotational sequence about the central axis to provide signal selectivity in a narrow beam width defined by the respective antenna units and whereby the selectivity is rotatable about the central axis, and
second mode scanning of the antenna units about the central axis in a received RF phase sensing mode within a least a selected one or more of the sectors operating the control system to provide identification which antenna unit within the selected sector is most closely representative of an azimuth representing the direction to the target source.
The preferred methodology includes alternating between the first and second modes.
More specifically, the methodology preferably involves scanning sectors of the array, wherein there is repetitive scanning about the complete circle of the array in the amplitude sensing mode to identify a maximum signal sector, then converting to said second mode scanning, and therein comparing received signal phase as between diametrically opposed pair antenna units until a closest possible phase comparison identifies which of the opposed pairs of log-periodic antennas is most closely aligned with the received signal.